This invention relates to a picture compression apparatus and a picture compression method suitable when used in an optical disc recording/reproducing apparatus.
Ordinarily, picture signals compression-encoded by the MPEG (Moving Picture Experts Group) system are recorded on optical disc. In the MPEG system, picture signals consisting of about 15 frames called GOP (Group-Of Picture) are caused to be set of processing units, and those picture signals are encoded every the GOP.
In one GOP, as shown in FIG. 1, there are I picture (Intra-Picture) (Intraframe encoded picture), P picture (Predictive-Picture) (Interframe forward direction predictive encoded picture), and B picture (Bidirectionally Predictive-picture) (Bidirectionally predictive encoded picture). I picture (picture I2) serves to maintain independence of GOP, wherein the entirety of that picture is encoded. P pictures (pictures P5, P8) are caused to undergo predictive encoding in forward direction from I picture or P picture. In this case, I picture and P picture are encoded in the same order as original picture. In addition, B pictures (pictures B0, B1, B3, B4, B6, B7) are caused to bidirectionally undergo predictive encoding from I picture or P picture.
Conversely, in the case of decoding compression-encoded picture signal, picture I2 is decoded by itself as shown in FIG. 1, but pictures except for the picture I2 are not decoded only by their own picture signals.
However, when the first half portion of GOP including I picture is missing, P picture or B picture of the latter half of that GOP fails to be decoded.
For example, picture signals encoded every GOP-1, GOP-2, GOP-3 are assumed to be recorded, as shown in FIG. 2, on recording medium (not shown). Let now consider the case where picture signals from picture B6 of GOP-1 until picture B3 of GOP-3 are rewritten into picture signals of new GOP-N.
As shown in FIG. 3, in the GOP-IX in which pictures B6, B7, P8 are missing, pictures until picture P5 including I picture are not missing. Accordingly, pictures except for the missing pictures are decoded in an ordinary manner.
However, in the GOP-3X in which pictures B0, B1, I2, B3 of the first half portion are missing, I picture of that first half portion is also missing. Accordingly, pictures except for missing pictures would also fail to be decoded.
Namely, in the case where when picture signals compression-encoded by the MPEG system are recorded on recording medium, compression-encoded other picture signals are further overwritten, there is not any problem if those picture signals are overwritten in GOP units. However, when picture signal of I picture of any other GOP is caused to be missing by the overwritten picture signals (in the case of units except for GOP unit), there took place the problem that as far as picture signals which are not missing fail to be decoded.
In order to avoid such problems, it is conceivable to rewrite picture signals every GOP. However, since those picture signals must be collectively rewritten every about 15 frames, such rewrite operation is very inconvenient. In addition, since picture signals are compression-encoded every respective one GOPs, it is impossible to rewrite those picture signals every pictures.
This invention has been proposed in view of such actual circumstances and its object is to provide a picture compression apparatus and a picture compression method which are capable of rewriting picture data in picture units in place of compression units.
In this invention, in the case where picture or pictures of (constituting) a portion of compression unit is or are missing, original compression unit in which the picture or pictures is or are missing is constructed as new compression unit to thereby solve the above-described problems.
Namely, a picture compression apparatus according to this invention is directed to a picture compression apparatus adapted for compressing picture data of plural pictures which correlate with each other to output compressed picture data every compression unit in which the plural pictures are compressed, the apparatus comprising: first compression means for compressing picture data to generate compression unit; and compression unit constructing means operative so that when picture or pictures of (constituting) the portion of the compression unit is or are missing, it constructs, as new compression unit, the original compression unit in which the picture or pictures of the portion thereof is or are missing.
It is preferable that, as the compression unit constructing means, there are provided expanding means operative so that when replacement into the different (new) compression unit is made so that picture or pictures of a portion of compression unit immediately after the different (new) compression unit is or are missing, it expands the original compression unit in which the picture or pictures is or are missing to generate original picture data, and second compression means for carrying out compression (of picture data of non-missing pictures of the original compression unit in order to expand (decode) the picture data of the non-missing pictures) by using the original picture data generated at the expanding means so as to construct the original compression unit in which the picture or pictures of the portion thereof is or are missing as new compression unit.
Further, it is preferable to alternately, in a time divisional manner, processing to expand, by the expanding means, original compression unit in which the picture or pictures of the portion thereof is or are missing to generate original picture data and processing to carry out compression (of picture data of non-missing pictures of the original compression unit in order to expand (decode) the picture data of the non-missing) by using the original picture data by the second compression means so as to construct the original compression unit in which picture or pictures is or are missing as new compression unit.
It is preferable that the expanding means carries out, with respect to the original compression unit, expansion at a speed higher than compression speed by the second compression means.
It is preferable that the second compression means compresses the original compression unit at a compression factor corresponding to data quantity of original picture data generated at the expanding means.
A picture compression method according to this invention is directed to a picture compression method of compressing picture data of plural pictures which correlate with each other to output compressed picture data every compression units in which the plural pictures are compressed, the method comprising: a first compression step of compressing picture data to generate compression unit; and a compression unit constructing step in which when picture or pictures of (constituting) the compression unit is or are missing, the original compression unit in which the picture or the pictures is or are missing is constructed as new compression unit.
It is preferable to comprise, as the compression unit constructing step, an expanding step in which when replacement into the different (new) compression unit is made so that picture or pictures of a portion of compression unit immediately after the different (new) compression unit is or are missing, the original compression unit in which the picture or pictures is or are missing is expanded to generate original picture data, and a second compression step of carrying out compression (of picture data of non-missing pictures of the original compression unit in order to expand (decode) the picture data of the non-missing pictures) by using the original picture data generated at the expanding step so as to construct the original compression unit in which the picture or pictures of the portion thereof is or are missing as new compression unit.
Further, it is preferable to alternately carry out, in a time divisional manner, processing to expand, by the expansion step, original compression unit in which the picture or pictures of the portion thereof is or are missing to generate original picture data, and processing to carry out compression (of picture data of non-missing pictures of the original compression unit in order to expand (decode) the picture data of the non-missing pictures) by using the original picture data by the second compression step so as to construct the original compression unit in which the picture or pictures of the portion thereof is or are missing as new compression unit.
It is preferable to carry out expansion, at the expansion step, with respect to the original compression unit, at a speed higher than compression speed by the second compression step.
It is preferable to compress, at the second compression step, the original compression unit at a compression factor corresponding to data quantity of original picture data generated at the expansion step.